Low power wireless devices are becoming more prevalent and sophisticated. These new wireless devices require very fast transfer of data while minimizing current drain from a self contained battery. Bandwidth Efficient methods for transmission of data have been and continue to be of great importance to the data dependent community. A community that is experiencing exponential growth. Minimum Shift Keying (MSK) is a type of continuous-phase frequency-shift keying that was developed in the late 1950s and 1960s, see U.S. Pat. No. 2,977,417, entitled “Minimum-Shift Data Communication System,” by Doelz et al., and incorporated by reference herein for all purposes. MSK is encoded with bits alternating between quadrature components, with the Q component delayed by half the symbol period. However, instead of square pulses as OQPSK uses, MSK encodes each bit as a half sinusoid. This results in a constant-modulus signal which reduces problems caused by non-linear distortion. In addition to being viewed as related to OQPSK, MSK can also be viewed as a continuous phase frequency shift keyed (CPFSK) signal with a frequency separation of one-half the bit rate.
Frequency modulation and phase modulation are closely related. A static frequency shift of +1 Hz means that the phase is constantly advancing at the rate of 360 degrees per second (2π radians/second) relative to the phase of the unshifted signal. Likewise, a static frequency shift of −1 Hz means that the phase is constantly retarding at the rate of −360 degrees per second (−2π radians/second) relative to the phase of the unshifted signal.
Since a frequency shift produces an advancing or retarding phase, frequency shifts can be detected by sampling phase at each symbol period. Phase shifts of (2N+1) π/2 radians are easily detected with an I/Q demodulator. At even numbered symbols, the polarity of the I channel conveys the transmitted data, while at odd numbered symbols the polarity of the Q channel conveys the data. This orthogonality between I and Q simplifies detection algorithms and hence reduces power consumption in a portable receiver. The minimum frequency shift which yields orthogonality of I and Q is that which results in a phase shift of ±π/2 radians per symbol (90 degrees per symbol). The deviation must be accurate in order to generate repeatable 90 degree phase shifts. A phase shift of +90 degrees represents a data bit equal to “1”, while −90 degrees represents a “0”. The peak-to-peak frequency shift of an MSK signal is equal to one-half of the bit rate.
The MSK signal produces a constant envelope carrier (no carrier amplitude variations) which is not affected by amplification through a higher efficiency non-linear amplifier, e.g., class C amplifier. It is a spectrally efficient modulation scheme (see GMSK below), has good bit error rate (BER) performance and is self synchronizing. These are all desirable characteristics for improving the power efficiency of receivers and transmitters, especially those operated from a battery. Amplitude variations can exercise nonlinearities in an amplifier's amplitude-transfer function, generating spectral regrowth, a component of adjacent channel power. Therefore, more efficient amplifiers (which tend to be less linear) can be used with constant-envelope signals, thereby reducing power consumption. MSK has a narrower spectrum than wider deviation forms of frequency shift keying (FSK). The width of the spectrum is also influenced by the waveforms causing the frequency shift. If those waveforms have fast transitions or a high slew rate, then the spectrum of the transmitter will be broad. In practice, the waveforms are filtered with a Gaussian filter, resulting in a narrow spectrum. In addition, the Gaussian filter has no time-domain overshoot that would broaden the spectrum by increasing the peak deviation. MSK in combination with a Gaussian filter is termed GMSK (Gaussian MSK).
IEEE standard 802.15.4-2006 (Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (WPANs), incorporated by reference herein for all purposes, defines the protocol and compatible interconnection for data communication devices using low-data-rate, low-power, and low-complexity short-range radio frequency (RF) transmissions in a wireless personal area network (WPAN) at data transfer rates of up to a 250 kilobit/second (kb/s). However with increased demand for content rich wireless data transfer, 250 kb/s is not fast enough. Up to a two (2) megabit per second (Mb/s) wireless data rate is desired as a selectable option for increased data transfer in a IEEE standard 802.15.4 compatible wireless device. With the constraints of a given analog radio, achieving this requirement would not be possible without advanced digital signal processing techniques.